The present invention relates in general to monitoring high voltages in electric vehicle drives, and, more specifically, to monitoring of battery contactor switches by a microcontroller that is digitally isolated from the high voltages without reliance on expensive switching/isolating devices.
The DC power source (e.g., a battery) and other elements of electric drives for hybrid or electric vehicles require monitoring in order to maximize efficiency and performance as well as to detect potential malfunctions. Common battery types such as lithium ion (Li-Ion) use a large number of cells stacked together into a battery pack generating hundreds of volts. It is very challenging to reliably monitor various conditions of the battery and electric drive system because of the high-voltage levels involved, the range of intermediate voltages at which respective cells operate within the stack, and the high levels of accuracy required.
Monitored voltages of the battery cells, battery pack, and associated devices are used by a main microcontroller or microprocessor for performing battery management and communication. Some of the primary components to be monitored include the battery contactor switches that couple the battery pack to the vehicle loads (e.g., the inverter for driving an electric motor). The main micro is typically located in a discrete battery control module or box that interfaces with other vehicle components such as a vehicle system/powertrain controller or a driver interface module. Consequently, the main micro uses a chassis ground for its voltage reference. The chassis ground is isolated from the main battery's reference which is provided at a negative battery bus.
Monitoring circuits deployed with the battery pack or elsewhere in the high-voltage system (e.g., the main contactors) are referenced to the negative bus. Since they are connected in the high-voltage domain, they must communicate with the main micro through domain-crossing elements that provide digital isolation between the high-voltage battery domain and the chassis ground domain (i.e., low voltage domain) of the main micro.
Monitoring functions such as verifying the state of a main contactor are usually not done on a continuous basis. Instead, a monitoring circuit is selectably switched on when the main micro needs to check on the contactor state. Intermittent checking reduces power consumption and reduces circuit interactions. Due to the high voltage environment, however, the semiconductor devices used for switching and isolation have been relatively expensive. For example, photoMOS switches and opto-isolators with relatively high voltage ratings and high prices have been required. It would be very desirable to avoid the use of such expensive components while maintaining robust detection of the state of the contactor switches, for example.